Help me with the MWI

I have one question about MWI, please help me find out the resolution.

According to the rfc3842, the subscriber send a SUBSCRIBE to server, the server will reply a NOTIFY to the subscriber for synchronizing state. The flow is:

       USER                                      SERVER
          |                                             |
          |          SUBSCRIBE                  |

Request |---------------------------------->|
| 200 reply |
|<----------------------------------|
| NOTIFY (sync) |
|<----------------------------------|
| 200 reply |
|---------------------------------->|
| |
| |
| NOTIFY (report) |
|<----------------------------------| Voice-mail changed.
| 200 reply |
|---------------------------------->|
| |
| |
| SUBSCRIBE |
Refresh |---------------------------------->|
| 200 reply |
|<----------------------------------|
| NOTIFY (sync) |
|<----------------------------------|
| 200 reply |
|---------------------------------->|

But when our gateway communicating with ASTERISK server, the server doesn?t send NOTIFY to the gateway when it received the SUBSCRIBE request. So it can?t synchronize the mail-voice state all the time. In some case, the MWI is failed. The flow is:

       USER                                    SERVER
          |                                            |
          |          SUBSCRIBE                 |

Request |---------------------------------->|
| 200 reply |
|<----------------------------------|
| |
| |
| NOTIFY (report) |
|<----------------------------------| Voice-mail changed.
| 200 reply |
|---------------------------------->|
| |
| |
| SUBSCRIBE |
Refresh |---------------------------------->|
| 200 reply |
|<----------------------------------|

Please help me, and give me your concern.
If you can, please give me the asterisk normtive mwi flow.
I am very appreciating for your reply.